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<H1><A ID="SECTION00050000000000000000">
4 Using <TT>PHonon</TT></A>
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<P>
Phonon calculation is presently a two-step process.
First, you have to find the ground-state atomic and electronic configuration;
Second, you can calculate phonons using Density-Functional Perturbation Theory.
Further processing to calculate Interatomic Force Constants, to add macroscopic
electric field and impose Acoustic Sum Rules at <IMG STYLE="height: 1.52ex; vertical-align: -0.55ex; " SRC="img1.png"
ALT="$\bf q$"> = 0 may be needed.
In the following, we will indicate by <IMG STYLE="height: 1.52ex; vertical-align: -0.55ex; " SRC="img1.png"
ALT="$\bf q$"> the phonon wavevectors,
while <IMG STYLE="height: 1.63ex; vertical-align: -0.10ex; " SRC="img3.png"
ALT="$\bf k$"> will indicate Bloch vectors used for summing over the
Brillouin Zone.
<P>
The main code <TT>ph.x</TT> can be used whenever <TT>PWscf</TT> can be used,
with the exceptions of DFT+U, hybrid functionals, external electric fields,
constraints on magnetization, nonperiodic boundary conditions.
USPP and PAW are not implemented for higher-order response calculations.
See the header of file <TT>PHonon/PH/phonon.f90</TT> for a complete and
updated list of what <TT>PHonon</TT> can and cannot do.
<P>
Since version 4.0 it is possible to safely stop execution of <TT>ph.x</TT> code using
the same mechanism of the <TT>pw.x</TT> code, i.e. by creating a file
<TT>prefix.EXIT</TT> in the working directory. Execution can be resumed by
setting <TT>recover=.true.</TT> in the subsequent input data.
Moreover the execution can be (cleanly) stopped after a given time is elapsed,
using variable <TT>max_seconds</TT>. See <TT>example/Recover_example/</TT>.
<P>
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<!--Table of Child-Links-->
<A ID="CHILD_LINKS"><STRONG>Subsections</STRONG></A>
<UL>
<LI><A ID="tex2html58"
HREF="node7.html">4.1 Single-<B>q</B> calculation</A>
<LI><A ID="tex2html59"
HREF="node8.html">4.2 Calculation of interatomic force constants in real space</A>
<LI><A ID="tex2html60"
HREF="node9.html">4.3 Calculation of electron-phonon interaction coefficients</A>
<LI><A ID="tex2html61"
HREF="node10.html">4.4 DFPT with the tetrahedron method</A>
<LI><A ID="tex2html62"
HREF="node11.html">4.5 Calculation of electron-phonon interaction coefficients with the tetrahedron method</A>
<LI><A ID="tex2html63"
HREF="node12.html">4.6 Phonons for two-dimensional crystals</A>
<LI><A ID="tex2html64"
HREF="node13.html">4.7 Phonons from DFPT+<I>U</I></A>
<LI><A ID="tex2html65"
HREF="node14.html">4.8 Fourier interpolation of phonon potential</A>
<LI><A ID="tex2html66"
HREF="node15.html">4.9 Calculation of phonon-renormalization of electron bands</A>
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